Gas turbine engines having multi-stage turbine sections typically have stator vanes placed at the entrance and exit of the turbine section, as well as between each turbine stage, for purposes of properly directing the air flow to each successive turbine stage. In service, stator vanes are exposed to a hostile environment which can erode the vanes and lead to the formation of undesirable cracks and voids in the surfaces of the vanes. Stator vanes may be repaired using a brazing operation in which a braze alloy is melted and flowed over the vane's surface in order to rebuild the damaged regions of the vane. As with brazing operations performed on many other types of components and machinery, the surface over which the molten braze alloy is permitted to flow must often be limited. For example, voids such as cooling, mounting and locating holes, seal grooves, and datum locators on the surface of a vane must not be filled by the molten braze alloy in order for these features to remain useful.
Presently, molten braze alloy is typically excluded from useful features present in the surface of a stator vane by the use of liquid braze blocking compositions. These compositions are typically composed of fine oxide particles, such as aluminum oxide (Al.sub.2 O.sub.3) or yttrium oxide (Y.sub.2 O.sub.3), suspended in a liquid carrier medium. Examples of this approach are taught by U.S. Pat. Nos. 2,473,887 to Jennings et al., 3,110,102 to Pfefferkorn, 3,623,921 and 3,906,617 to Behringer et al., 3,858,303 to Horbury et al., and 4,023,251 and 4,040,159 to Darrow. The liquid carrier is generally formulated such that it can be either evaporated prior to brazing or burned off during the brazing process, so as to leave a cohesive film of oxide powder on the surface of the vane. If the oxide powder film is sufficiently thick, the braze alloy will not flow onto or adhere to the film due to surface tension effects.
A disadvantage with this type of braze blocking composition arises when a crack or void to be repaired intersects one of the required features in the vane, such as a cooling hole. As such, in an attempt to fill the hole with the braze blocking composition, the blocking composition will also tend to flow into and fill at least a portion of the void to be repaired, due to the composition being in the form of a liquid or slurry. As a result, the braze alloy is prevented from adequately filling and/or adhering to the surface of the undesirable void, or at best will result in a porous braze fill due to an interaction between the braze alloy and the braze blocking composition. Consequently, the time required for repairing and refurbishing stator vanes can be significant due to the requirement for post-brazing inspections, repairs and scrappage, all of which add considerable processing and material costs.
While the prior art has suggested the use of braze blocking pastes, such as those taught by U.S. Pat. Nos. 3,846,903 to Rupert et al. and 4,634,039 to Banerjee, the accuracy with which these pastes may be applied is often inadequate for repairing and refurbishing stator vanes. Even if precisely applied, known braze blocking compositions tend to shrink as the liquid carrier is evaporated or volatilized, such that molten braze alloy is permitted to flow into the voids, holes and grooves which are intended to remain open.
Accordingly, it would be advantageous to provide an improved braze blocking composition which overcomes the shortcomings of the prior art. Specifically, it would be desirable to provide a braze blocking composition which can be precisely placed to completely and reliably fill a void, hole or groove on the surface of an article, such as the cooling holes and seal grooves of a stator vane for a gas turbine engine, such that molten braze alloy will not flow into the void, hole or groove during a brazing operation. In addition, it would be advantageous if such a braze blocking composition could be readily removed after brazing.